There are two main parts of this program project. The first part consists of a longitudinal, multidisciplinary study of 200 preschool aged children (75 with autism, 50 with mental retardation, and 75 with typical development). Projects I-IV are each a component of this longitudinal study, focusing on one of the following domains of early development: Project I: social cognition and patterns of functional electrophysiological brain imaging in response to social stimuli; Project II: Precursors of speech and language, and patterns of functional electrophysiological brain imaging to speech stimuli; Project III: Developmental course during early infancy, including variations in age of symptom onset; Project IV: Brain structure and metabolism, as assessed by magnetic resonance imaging and spectroscopy. Individual differences in each domain will be related to concurrent measures of neuropsychological functioning and will be used to predict response to intervention and outcome in autism at 6-7 years of age. The second major part of the program project is a collaborative genetic linkage study of sibling pairs with autism (Project V). Information gained in the longitudinal study (Projects I-IV) will be used to empirically-validate neurobiological subtypes of autism, and this information will be subsequently used to model genetic heterogeneity. Thus, all five research projects iwll function as a whole in an integrated, synergistic way to offer a deeper understanding of the neurobiological and genetic basis of autism. Core A will provide administrative support, and be responsible for diagnostic and neuropsychological evaluations. Core B will provide statistical and technical support to the projects.